1. Field of the Invention
The present invention relates to a vehicle bumper and a method of making the same. The invention particularly relates to a one-piece, blow-formed, hollow bumper having an integral reinforcement frame and a method of making the same.
2. Description of the Prior Art
Until recently bumpers employed in vehicles such as automobiles and the like have been metallic. However, in recent years, mainly bumpers made from resin composite materials have been employed to meet weight reduction demands. In oder to impart sufficient rigidity, many composite resin bumpers are reinforced with metallic reinforcements. An example of a bumper in which sheet metal reinforcements are formed integrally into the rear of a main bumper body made of hollow composite resin material is disclosed in Japanese Utility Model Unexamined Publication No. 58-194941. In the described example, the reinforcements are closely matched and extend across substantially the entire width of the rear of the main body of the bumper. The reinforcements have pins on their upper and lower parts which project in the forward direction and are integrated into the main body of the bumper by insertion. As a result, when the long reinforcements are integrated into the main bumper body, a problem arises in that because of the difference in thermal expansion coefficients, peeling off and play between the frame and the bumper occurs.
This type of problem is solved, for example, by providing the possibility of relative movement in a separately formed composite resin main bumper body connected to the metal plate reinforcements by means of a nuts and bolts, as is described in Japanese Utility Model Unexamined Publication No. 62-78543.
In order to sufficiently protect the vehicle and the passengers, the bumper must have sufficient initial rigidity and load tolerance. After all, in a crash exceeding a given limit, in order for the seat belt and/or air bag, which protects the passenger, to function well, there are great demands on the bumper to keenly tolerate high loads without crushing. Also, it is desirable that the bumper body itself has sufficient load tolerance to crushing in order to protect the vehicle body from damage.
In order to obtain sufficient initial rigidity, it is necessary for the bumper to be strengthened by being formed integrally with reinforcements, for example, by being formed in one piece by means of blow forming. However, as stated above, if peeling and/or play between the frame and the bumper occurs, not only does the initial rigidity decrease, but so does the load tolerance. In addition, if, as in the prior art, a hollow bumper body is integrated with metallic plate reinforcements in its rear surface, because the reinforcements are fixed to the vehicle body by means of so-called stays, a design problem results because the total length of the vehicle is increased. One the other hand, if, as subsequent artisans have done, relative movement of the reinforcements is provided in the main body of the bumper, it is difficult to obtain sufficient initial rigidity. If the bumper has a large camber, the decrease in initial rigidity is particularly severe, because the stretching deformation of the bumper in the left and right directions is large during impact.